''Practically perfect in every way,'' Mary Poppins flies out of the windy London skies and into the home of two mischievous children. With the help of a carefree chimney sweep named Bert, the spirited nanny turns every chore into a game and every day into a ''Jolly Holiday.'' Also starring David Tomlinson, Glynis Johns, Ed Wynn, Jane Darwell, Karen Dotrice and Matthew Garber.